MRI apparatus

ABSTRACT

An MRI apparatus including a magnetic structure defining a cavity for receiving a body under examination or a part thereof, a mechanism for generating a magnetic field inside the cavity, a mechanism for causing the body under examination or a part thereof to emit nuclear magnetic resonance signals, and a mechanism for receiving the nuclear magnetic resonance signals. The mechanism for generating the magnetic field includes one or more elements made of permanently magnetized material of the so-called superconducting bulk material type and, in combination therewith, a mechanism for keeping the magnetization condition of the superconducting bulk material which includes mechanisms for maintaining the temperature of the permanently magnetized material below the critical temperature thereof and for restoring the magnetization of the superconducting bulk material upon a complete or partial demagnetization.

The present invention relates to a MRI apparatus comprising a magneticstructure delimiting a cavity receiving a body under examination or apart thereof, and comprising means for generating a magnetic fieldinside said cavity, as well as means for causing the body underexamination or a part thereof to emit nuclear magnetic resonance signalsand means for receiving said nuclear magnetic resonance signals,

the MRI apparatus further comprises an electronic processing unit saidmeans receiving the nuclear magnetic resonance signals beingelectrically connected thereto;

the cavity having at least an access opening through which a body underexamination or a part of said body may enter said cavity.

Currently there are two types of MRI apparatuses that are different onewith respect to the other as regards the size of the receiving cavityand as regards the strength of the static magnetic field of the nuclearspin orientation. Said two types of apparatuses are called as “totalbody” apparatuses or “dedicated” apparatuses.

Dedicated apparatuses are smaller and it is easier to install them,since both their size and weight are reduced, therefore it is notnecessary to prepare receiving spaces having suitable dimensions andsupporting structures fit for the load.

The receiving cavities of the “dedicated” apparatuses have such a sizeallowing only specific parts of a body and only some anatomicaldistricts thereof to be received and so to be examined. As regards suchapparatuses, typically the static magnetic field is generated bypermanent magnets not requiring complex, cumbersome and expensivecooling systems that on the contrary are necessary for high-field andtotal body apparatuses operating by the use of superconducting coils.

However current dedicated apparatuses are still quite cumbersome, sinceeven in the case of a static magnetic field with a medium-low strengthit is necessary to have a relatively large amount of magnetized materialto reach said strength levels and the magnetic structure needs asupporting structure and further elements such as possible yokes and/orpossible ferromagnetic plates. Therefore the reduction of the size ofthe magnetic structure and of the MRI apparatus is not as much great asthe reduction of the receiving cavity for being used only by specificparts of a body. Moreover the medium-low static magnetic field causesthe apparatus to be more susceptible to external noise fields, sincesignals emitted from the body under examination have a relatively lowintensity.

The use of conventional methods and materials for generating the staticmagnetic field, both with permanent magnets, resistive magnets andsuperconducting magnets, is therefore a restraint of the furtherreduction of the size and weight of MRI apparatuses and above all ofdedicated apparatuses.

Therefore currently there is an unsatisfied need for reducing the sizeof MRI apparatuses, and particularly of dedicated MRI apparatuses,without considering the fact of further reducing the strength of thestatic magnetic field. Moreover on the contrary, the strength of thestatic magnetic field is highly desired to be increased for the samesize especially as regards dedicated MRI apparatuses.

Particularly from the document “Construction of various types of strongmagnetic field generators using superconducting bulk magnets”Superconductor Science and Technology 18 (2005) S72-S76 di T Oka1, KYokoyama1 and K Noto, published on 15 Dec. 2004 (available on line athttp://stacks.iop.org/SUST/18/S72) magnets made of superconducting bulkmaterial are known, wherein magnetization currents are trapped andfirmly frozen. The elements made of said material when kept below thecritical temperature maintain the magnetization and behave likeconventional permanent magnets.

The above document deals with the study of the above mentioned effectwith a particular reference to the so called type-II or high criticaltemperature superconducting materials. However it is known that sucheffect occurs also with low critical temperature superconductingmaterials, that is the so called type-I superconducting materials.

In the above mentioned document the magnetic field generated by suchsuperconducting bulk materials varies extremely throughout the surfaceof the element made of said material and therefore at a first sight itseems not to fit the purpose.

The document U.S. Pat. No. 7,498,915 discloses an annular magnet made ofsuperconducting material that is permanently magnetized. The fact ofmanufacturing the magnet with magnetized elements of superconductingbulk material which form the annular structure allows drawbacks relatedto the homogeneity to be overcome, but the geometry of the magnet isbound by the closed annular shape and so the size of the magnet cannotbe reduced as desired and cannot be adapted for example to the geometryof a part of the body it being possible to exclusively dedicate a MRIapparatus to the examination thereof.

Moreover in said document the magnetization and the production of theelements made of superconducting material is complex and rather obscure.Indeed it provides to separately magnetize individual particles made ofsuperconducting material and then said particles are assembled togetherto form trapezoidal elements that once joined together make the annularstructure. How technically assembling together the magnetized particlesis not explained above all if considering that particles have to be keptbelow the critical temperature in order not to lose the magnetizationsimilar to the one of permanent magnets and therefore the fact ofassembling them together into the individual elements seems not possiblewithout using processes requiring high temperatures, such as sinteringand/or fusion processes.

The teaching of the document U.S. Pat. No. 7,498,915 is therefore veryrudimentary as regards the production of a magnetic structure that canbe actually manufactured and used in a practically operating MRIapparatus.

Moreover within both the above mentioned documents, there is a precisereference only to the use of high critical temperature superconductingmaterials. This leads to limits both related to the relatively highcosts of said materials, and related to the resistance of saidmaterials, which are subjected to a disaggregation with high magneticfields since they are relatively porous, therefore when manufacturingpermanent magnets made of type-II superconducting material they have tobe coated, i.e. they have to be enclosed into an evelope or shell madeof metallic material such as steel or the like.

The tendency to limit the use of high critical temperaturesuperconducing materials is due to the fact that for cooling saidmaterials below the critical temperature it is sufficient to use liquidnitrogen, while for cooling low critical temperature superconductingmaterials it is necessary to use cooling materials with liquid helium orthe like. However, considering the field strength that can be reachedwith permanent magnets made of superconducting bulk materials and thereduction of the size of the magnet for dedicated apparatuses thecooling process has no considerable problems even at lower temperature,since the mass of the superconducting material necessary to achieve thedesired field strength and the volume of said mass are relativelylimited.

The invention aims at providing a MRI apparatus of the type describedhereinbefore which apparatus by means of inexpensive and simplearrangements can be provided with a very small size and substantiallycorresponding to the size of at least a part of a body said apparatus issubstantially solely intended to examine, without considering excessivelimitations of the static magnetic field and without using inductioncurrents for generating said static magnetic field.

The invention achieves the above aims by providing a MRI apparatuscomprising a magnetic structure defining a cavity for receiving a bodyunder examination or a part thereof, and comprising means for generatinga magnetic field inside said cavity, as well as means for causing thebody under examination or a part thereof to emit nuclear magneticresonance signals and means for receiving said nuclear magneticresonance signals,

the MRI apparatus further comprises an electronic processing unit saidmeans receiving the nuclear magnetic resonance signals beingelectrically connected thereto;

the cavity having at least an access opening through which a body underexamination or a part of said body can enter said cavity

and wherein means for generating the magnetic field comprise one or moreelements made of permanently magnetized material of the type so calledsuperconducting bulk material and i.e. a material wherein magnetizationcurrents are firmly trapped, in combination with said elements made ofpermanently magnetized material there being provided means for keepingsaid permanent magnetization condition of said superconducting bulkmaterial.

Said means alternatively or in combination one to the other can becomposed of means for keeping the temperature of said permanentlymagnetized material below the critical temperature thereof and means formagnetizing said superconducting bulk material which can be operatedwhen the magnetization condition changes and/or upon the demagnetizationof said superconducting bulk material.

The permanently magnetized superconducting bulk material can be both ofthe type-I and type-II and particularly said superconducting bulkmaterial is composed of a medium critical temperature material andparticularly of MgB₂.

Medium critical temperature materials mean materials with a criticaltemperature from 20 to 60, preferably 35 to 50 kelvin degrees.

The use of such materials is advantageous since unlike low criticaltemperature materials they do not need the use of liquid helium (4.2° K)which requires means for handling the liquid helium not only forreaching and keeping the liquefaction temperature, but also for safetyreasons, since a reduced amount of liquid helium once back in thegaseous condition can cause the percentage of oxygen to change per unitof air volume such to represent a risk of suffocation. For this reason aMRI apparatus using liquid helium requires such constructioncharacteristics to be safe even as regards such point of view andtherefore it has an expensive, complex and cumbersome construction.

On the other hand high critical temperature superconducting materialshave very porous structures and so they tend to be disintegrated underthe action of the magnetic field trapped therein, so they are oftenenclosed into metal envelopes.

In the case of medium critical temperature materials, they can be cooledbelow the critical temperature by means of gases having higherliquefaction temperatures with respect to helium and possibly also bymeans of thermal contact cooling devices called cryo-coolers.

Since in the case of a rise in the temperature above the criticaltemperature, for example due to a malfunction of means cooling thesuperconducting bulk material, the latter loses the magnetizationcondition, the invention provides in addition means for magnetizing themagnetized material which can be mounted in a stationary way into themagnetic structure and in such a relative position to magnetize at leasta part of the superconducting bulk material or as an alternative theycan be of the removable type the magnetic structure and/or the poles andthe magnetization means being provided with mutually removable fasteningmeans.

In the case of removable magnetization means, the removable means formutually fastening the magnetization means to the magnetic structureand/or to the poles are composed of a combination of sliding guides andslides or saddles which are slidably engaged one to the other accordingto an insertion/extraction direction of the magnetization means in aposition coupling with the corresponding pole.

Advantageously and unlike the known annular structure, the apparatusaccording to the present invention has at least two magnetic poles,arranged at the opposite sides of the receiving cavity and in aspaced-apart relation substantially corresponding to the dimension inthe direction of said distance between the two poles, each magnetic polebeing composed at least partially of a superconducting material which ismagnetized and kept at a temperature below the critical temperature ofsaid material.

According to a preferred embodiment, the MRI apparatus comprises amagnetic structure with a yoke and two magnetic poles, each magneticpole comprises in turn a layer of magnetized material in the form of aplate or sheet and the magnetic poles are parallel and opposite one tothe other and are formed at two parallel and opposite sides of the yoke,while the magnetizations of the layers of magnetized material beingparallel and aligned in the same direction, substiantially perpendicularto the inner faces of the main magnetic poles, said magnetic structurebeing further shaped such to define or enclose a cavity, at least aportion of the volume of said cavity being a space for receiving atleast a part of a body under examination, at least a portion of saidcavity being permeated by a static magnetic field generated by themagnetic poles and having specific strength and homogeneitycharacteristics and said magnetic structure being open at least one sideparallel to the static magnetic field.

With reference to the construction of the removable magnetization means,the sliding guides for the insertion/extraction of the magnetizationmeans are oriented parallely to the axis of the or one of the accessopenings of the cavity and they are provided in a position coincidentwith the passage span defined by said opening such that saidmagnetization means are inserted/extracted in and from the positioncoupling to the permanently magnetized material through said accessopening.

The yoke can be replaced also by a supporting structure with polesgenerating the magnetic field secured thereto, which structure does notserve at all for generating the static magnetic field or for guiding thestatic magnetic field.

According to a further advantageous characteristic, each element made ofmagnetized material is in the form of a layer like a sheet or plate madeof permanently magnetized material having a predetermined thickness anda predetermined area and a predetermined plan shape.

In this case, the guides for inserting/extracting the magnetizationmeans are parallely oriented with respect to the surface of said sheetor plate of permanently magnetized material at the side of said sheet orplate and in such a position that with the magnetization means in theinserted condition into said guides, said means overlap the surface ofsaid sheet or plate made of magnetized material.

According to a possible construction variant, on the side faced towardsthe cavity each sheet or plate made of magnetized material provided foreach magnetic pole is overlapped by a sheet-like or plate-like elementmade of ferromagnetic material.

When the plan dimension of the layer of superconducting bulk material inthe form of a sheet or plate is relatively large, it is advantageous foreach sheet or plate made of permanently magnetized material to becomposed of at least a layer of blocks or cards made of superconductingbulk material arranged one near the other one into a two-dimensionalarray of blocks or cards made of superconducting bulk material, meansfor individually magnetizing each block or each card of superconductingbulk material being provided.

As regards means for keeping the temperature of the individual blocks orindividual cards of superconducting bulk material below the criticaltemperature they can be separated for each block or each card or saidmeans for maintaining the temperature are in common to all the blocks orto all the cards of one sheet or plate made of permanently magnetizedmaterial.

As an alternative, each sheet or plate of permanently magnetizedmaterial can be in the form of one piece and has a side faced towardsthe cavity which side has a predetermined plan shape and a predeterminedarea, said area being ideally divided into a plurality of adjacentsectors like an array of sectors and each one of said sectors beingmagnetized separately from the other sectors.

In both the cases mentioned above, the magnetization means can becomposed of a supporting frame that can be inserted/extracted by theinsertion/extraction guides and upon which frame there are mounted meansfor supporting a magnetization head having such a size to cooperate onlywith one of the blocks or cards forming a sheet of superconducting bulkmaterial or only with one of the sectors into which the area of a faceof the sheet or plate in the form of one piece of superconducting bulkmaterial is ideally divided respectively and it can be moved in twoperpendicular directions alternatively in one position cooperating witha block or a card or with a sector one at a time respectively of thesheet or plate in the form of one piece of superconducting bulk materialof the plurality of blocks, cards or sectors.

As regards the construction it is possible to provide an embodimentwherein the frame has a first slide slidable along one of twoperpendicular directions, which slide is the supporting guide sliding asecond slide that is movable along said first slide in said seconddirection.

As an alternative to the previous solution and still for the two caseswith a sheet or plate of superconducting bulk material in the form ofone piece or composed of individual adjacent blocks or cards, themagnetization means can be composed of an assembly of individualmagnetization units, each one of them having such a size to magnetize ablock, a card or a sector forming the sheet or plate of superconductingbulk material said magnetization units being mounted one with respect tothe other on a supporting frame according to an order and an arrangementof the relative positions one with respect to the other corresponding tothe relative arrangement one with respect to the other of blocks, cardsor sectors forming the sheet or plate of superconducting bulk materialand such that each one of said magnetization units can be taken in aposition cooperating with a corresponding block, a corresponding card ora corresponding sector simultaneously to the other magnetization unitsand during a single and common motion inserting the assembly of themagnetization units into the insertion or extraction guides.

In this case, each magnetization unit can be operated separately fromeach other, said units being simultaneously operated at different timesaccording to a predetermined sequence.

By means of the above combinations of characteristics each block, eachcard or each sector of the plate or sheet of superconducting bulkmaterial can be magnetized such to achieve a different magnetization onefrom the other for generating a predetermined arrangement of themagnetic field throughout the area of said plate or sheet.

This is very important for achieving a static magnetic field with a highhomogeneity between the two magnetic poles and into the cavity sinceeach block, each card or each sector can be magnetized such to optimizethe homogeneity conditions of the static magnetic field into the cavity.

Moreover the different magnetization can also be used to compensatesystematic aberrations of the static magnetic field provided at the opensides for entering the cavity.

Further improvements are object of the subclaims.

Characteristics of the invention and advantages deriving therefrom willbe more clear from the following description of some non limitativeembodiments shown in annexed drawings, wherein:

FIG. 1 is a first embodiment of a magnetic structure for generating thestatic magnetic field into a cavity receiving at least a part of a bodyof a patient, which structure has an annular or tubolar cross-section.

FIG. 2 is a variant embodiment of the magnetic structure according tothe precedeing FIG. 1 providing in combination means for magnetizing thesuperconducting bulk material of magnetic poles which are of the typethat can be mounted and removed from the magnetic structure.

FIG. 3 is a schematic enlarged view of the magnetization means accordingto FIG. 2 and of the removable supporting means into the cavity of themagnetic structure.

FIG. 4 is a side view of a variant embodiment of the magnetic structureaccording to the previous figures wherein the structure has three opensides, that is it has a C-shaped or overturned U-shaped cross-section.

FIG. 5 is a front view of the structure according to FIG. 4 taken alonga vertical plane parallel to the vertical closed side and intersectingthe two horizontal and parallel branches of the structure in anintermediate position between the main poles and the secondary poles.

FIG. 6 is a variant embodiment of the magnetic structure according toFIGS. 1 to 3 wherein the magnetization means are firmly mounted thereinat the side of the poles faced towards the supporting structure ortowards the yoke and opposite to the receiving cavity, into an housingrecess.

FIGS. 7 to 11 schematically are different variants of the combination ofa magnetic pole comprising at least a layer of superconducting bulkmaterial and of magnetization means associated thereto allowing the faceof said layer of superconducting bulk material faced towards thereceiving cavity to be divided into sectors each sector being magnetizedor magnetizable individually and differently from the other sectors.

FIG. 12 is an alternative variant embodiment of the combinationsaccording to FIGS. 7 to 11, wherein the magnetization means work on asingle sector at a time and they can be moved from one sector to anotherone of the face of the layer of magnetized material.

FIGS. 13 and 14 like FIGS. 4 and 5 are a C-shaped or overturned U-shapedmagnetic structure wherein combinations of magnetic poles andmagnetization means are mounted according to some variants of FIGS. 7 to12, the upper and lower poles being different one with respect to theother.

FIG. 15 is a top plan view on the face of a layer of superconductingbulk material faced towards the receiving cavity.

With reference to FIG. 1, there is schematically shown a magneticstructure of a MRI apparatus i.e. for nuclear magnetic resonancediagnostic imaging, which structure comprises a supporting structure 1which in the shown embodiment has an annular or tubular cross-sectionand it encloses on four sides a cavity for receiving at least a part ofthe body of a patient. In the most simple embodiment the supportingstructure has a quadrangular cross-section and it is composed of fourplates which are connected together at their ends such to form the fourcorner areas of the quadrangular annular cross-section. There are twovertical plates denoted by 201 and two horizontal plates denoted by 101.While the shape of the shown supporting structure is the most simple oneit is possible to provide also other shapes.

With reference to the center of the cavity delimited by said structure 1at the two opposite sides it bears two magnetic poles generally denotedby 2 and intended for generating a static magnetic field called Bo whichpermeates the receiving cavity from one pole to the other one.

The two poles are substantially parallel one with respect to the otherand are spaced apart being arranged in a coincident position.

As usual in addition to the magnetic structure the MRI apparatuscomprises also means for causing the body under examination or a partthereof to emit nuclear magnetic resonance signals and means forreceiving said nuclear magnetic resonance signals and an electronicprocessing unit said means receiving the nuclear magnetic resonancesignals being electrically connected thereto. Such means are not shownin details since they are a known integrating part of MRI apparatuses.

The receiving cavity comprises at least an access opening through whicha body under examination or a part of said body can enter said cavityand particularly in the shown embodiment two access openings which areopposite and aligned with reference to an axis perpendicular to the spanof said two openings.

This arrangement is known and it is described in several documents suchas for example in EP921408 to the same applicant.

Magnetic poles are composed of at least a layer of magnetized material202 in the form of a plate or sheet and which is supported by one of theplates 101 constituting the supporting structure 1. The magnetizedmaterial is of the so called superconducting bulk type and itstemperature is kept below the critical temperature by means of coolingmeans 102. The latter are known and widely used as regardssuperconducting magnets which are used also within MRI apparatuses andtherefore they are not described and shown here in details.

It is possible to provide a layer made of permeable material on thelayer 202 of magnetized material such as for example ferromagneticmaterial in the form of a plate or sheet denoted by 302. It issuperposed to the face of the layer 202 made of superconductingmagnetized material faced towards the receiving cavity. The plate orsheet may be made of solid material or may be laminated or a portion maybe made of solid material and a portion may be laminated. The sheet orplate made of permeable material can be also omitted.

As already described above, the layer of magnetized material is composedof superconducting bulk material. Such materials are known and by meansof magnetization sources it is possible to generate internalmagnetization currents therein which are kept as long as the material iskept below the critical temperature.

Magnetization methods and materials suitable for being used as permanentmagnets made of superconducting material are known and are disclosed forexample in U.S. Pat. No. 6,111,490 and U.S. Pat. No. 6,441,710 whereinthe magnetization occurs by applying an external pulsed magnetic fieldgenerated by an external resistive or superconducting magnet to a solidelement made of superconducting material which is cooled below thetransition temperature (critical temperature).

Further similar alternatives are described in the following documents:Superconducting Permanent Magnet Made by a bulk MgB2 Ring. G. Giunchi etal. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY VOL. 18. NO 2 June2008; A Persistent-Mode Magnet comprised of YBCO annuli, Yukikazu Iwasaet al. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY VOL. 15. NO 2 June2005; A Novel heat engine for magnetizing superconductors, T. A. Coombset al. SUPERCONDUCTOR SCIENCE AND TECHNOLOGY 21 (2008) 034001 IOPPUBLISHING 2008, construction of strong magnetic fields generators byhigh Tc bulk superconductors and its applications, IEEE TRANSACTIONS ONAPPLIED SUPERCONDUCTIVITY VOL. 14. No. 2 June 2004.

A magnetization method for bulk superconducting materials which isparticularly advantageous and relatively simple as regards theconstruction is disclosed in WO2007/045929. Here, any type ofsuperconducting material i.e. with a high or low critical temperature issubjected to “permanent” magnetization, with said material being in thecooled condition below the critical temperature and by the generation ofwaves changing the magnetic flux through said material. The change inthe magnetic flux is achieved by providing at least a material changingits magnetic characteristics depending on the temperature and bychanging the temperature of said material such to alternatively generatea change in the magnetic characterstics thereof. The documentWO2007/045929 has to be considered as a part of such disclosure.

Therefore the magnetization of the layer made of superconducting bulkmaterial may be kept merely by the fact that said material and thereforethe layers made of it are kept at a temperature below the critical oneand wherein magnetization currents are “frozen” inside the material. Arise in the temperature causes superconductivity conditions to bechanged and so causes characteristics of the generated magnetic field tobe changed and if such changes cause the temperature to rise above thecritical temperature, the material loses its magnetization condition.

As it results from prior art materials that are currently preferablyused for making permanent magnets made of superconducting bulk materialuse materials having a high critical temperature, i.e. about 70° K. Thisis advantageous since the critical temperature is such to allow thetemperature to be kept below the critical temperature of thesuperconducting bulk material simply by using liquid nitrogen. The factof reaching such temperature does not require extreme technical effortsand it does not imply high costs. However materials with a high criticaltemperature are materials with a high granulometry and are of theceramic type thus with high magnetic fields the stress on the materialcan cause it to be disgregated. To this aim materials with a highcritical temperature have to be enclosed into envelopes made of moreresistant metallic material preventing constructional parts made of suchmaterials to be disgregated when the magnetization exceeds predeterminedvalues.

Therefore as regards the layer made of magnetized material the inventionprovides to use permanent magnets made of superconducting bulk materialof the type having a medium critical temperature namely ranging from 20to 60, preferably from 35 to 50 Kelvin degrees.

The use of such materials is advantageous since unlike low criticaltemperature materials they do not need the use of liquid helium (4.2° K)which requires means for handling the liquid helium not only forreaching and keeping the liquefaction temperature of helium, but alsofor safety reasons. Indeed a reduced amount of liquid helium, once backin the gaseous condition, can cause the percentage of oxygen to changeper unit of air volume such to represent a risk of suffocation. For thisreason a MRI apparatus using liquid helium requires such constructioncharacteristics to be safe even as regards such point of view andtherefore it has an expensive, complex and cumbersome construction.

In the case of medium critical temperature materials, they can be cooledbelow the critical temperature by means of gases having higherliquefaction temperatures with respect to helium and possibly also bymeans of thermal contact cooling devices called cryo-coolers.

The manufacturing burden of such magnets is greater than the requiredone if high critical temperature material is used, however the coolingtechnology for medium critical temperatures has reached considerabledevelopment stages and both the fact of obtaining liquid gases suitablefor the desired temperature range and the management of such liquid gasfor controlling the temperature are not serious technical problems andthey do not lead to too high costs.

Surprisingly such materials allow static magnetic field with a highfield strength and having a sufficient homogeneity to be achieved withinmagnetic structures that are very simple and with substantially only twoopposite magnetic poles 2 with a simple plate- or sheet-like shape thatas regards the construction are similar to the magnetic structures withmagnetic poles comprising conventional permanently magnetized materials.For the same overall dimension of the magnetic structure and so of thefinal apparatus and for the same weight thereof, the static magneticfield that can be obtained is at least for a factor from 2 to greaterthan the one of an apparatus having a magnetic structure comprisingconventional permanent magnets.

As it is clear from what said above, in the case of magnetic structureswhose poles comprise permanent magnets made of superconducting bulkmaterial, it is very important to monitor the temperature of themagnetized superconducting material for causing a MRI apparatus tooperate properly. A rise in the temperature can cause the generatedstatic magnetic field strength to change as long as it is below thecritical temperature or it can cause even the magnetization to be ceasedwhen the temperature rises above the critical one.

To this aim, according to a first advantageous characteristic themagnetic poles 2 described above and having one of the alternativeconstructional structure are firmly or removably associated tomagnetization means which are operatively arranged with respect to themagnetic poles 2, such that they can be used for a new magnetization orfor adjusting the magnetization condition of the magnetized ormagnetizable superconducting material constituting said magnetic poles.

As it results from the above mentioned prior art there are severalmagnetization methods and each one of such methods requires theconstruction of the magnetic structure to be partially modified howeverthis is known to the person skilled in the art once magnetization meansto be used are defined.

FIG. 2 shows a first embodiment of the magnetization means 5 which maybe removably mounted within the magnetic structure 1 in such a positionthat the superconducting bulk material 202 of each one of the at leasttwo poles 2 can be magnetized, once such material has partially orcompletely lost the magnetization for example due to a temperature riseor to a rise of said temperature above the critical temperature of saidsuperconducting bulk material constituting the plate or sheet 202 madeof magnetized material respectively.

With reference to such embodiment, the magnetization means 5 which areschematically shown are the same disclosed in WO2007/045929. Thisdocument describes the magnetization as being achieved by the passage ofthermal waves which change a pulse of magnetic disordering in anotherwise ordered material by heating said material to above the Curiepoint thereof.

The embodiment of the magnetization means provided in such example canbe anyone of the embodiments described in WO2007/045929 and particularlyin the shown example magnetization means are in the form of acombination of layers and they comprise a layer 305 of magnetizablematerial which is arranged as close as possible to the superconductingbulk material of the layer 202 of magnetized material of thecorresponding magnetic pole 2 and at least a temperature changingelement (heater or cooler) 205 that is driven by control signals inorder to change the temperature of the layer 305 of magnetizablematerial and so to generate, thermal waves that in turn change themagnetic order or disorder within the magnetizable material and thusthey generate magnetic flux changes in the superconducting material.

Moreover said magnetization means 5 at the magnetic poles 2 compriseremovable mounting means for being mounted in the position formagnetizing the layer 202 made of superconducting bulk material insidethe receiving cavity and through the opening or openings for enteringsaid cavity.

Such means generally denoted by 4 in FIG. 2 can be of any type andadvantageously they are composed of side support guides 104 mountedalong the opposite walls 201 perpendicular to the surfaces of themagnetic poles 2, particularly to the surface of said poles facedtowards the center of the receiving cavity. Said guides 104schematically shown are sliding guides and define a sliding plane forinserting and extracting the magnetization means which is substantiallyparallel to the surface of the corresponding magnetic pole 2 facedtowards the center of the receiving cavity and they are intended tohouse slides 204 provided along the corresponding side edges of themagnetization means 5 made as a plate composed of overlapped layers ofdifferent functional elements such as the ones 305 and 205 describedabove, while the thickness of the magnetization means 5 and the positionof the guides 104 and slides 204 with respect to the surface of thecorresponding magnetic pole 2 is such that, the magnetization means byone of their surfaces overlap the surface of the corresponding magneticpole 2 faced towards the center of the receiving cavity grazing it orwith such a distance as small as possible, said two surfaces being keptfaced and parallel one to the other or being leant one against theother.

From the above the advantages of the removable magnetization means 5 areclear. Particularly the embodiment providing magnetization means 5 thatcan be extracted by being parallely translated with respect to thesurface of the corresponding magnetic pole 2 faced towards the center ofthe receiving cavity and which insertion/extraction direction isperpendicular to an opening for entering the receiving cavity requiresthe magnetization means 5 to be subjected to a manual operation or to anoperation by mechanical means which is simple and which can be carriedout by simple construction means. There is the further advantage ofmaking the magnetization means 5 with an operative magnetization surfaceintended to cooperate with said surface of the magnetic pole 2 parallelthereto and to the insertion/extraction direction. The technical effectis further improved by making the magnetization means 5 like amulti-layered plate and this embodiment can be achieved in a simple wayas regards the contruction by using magnetization means of the typedescribed in the document WO2007/045929.

Again according to a further embodiment, whose characteristics areschematically shown in FIG. 3, control cables and/or cables supplyingpower to the magnetization means 5 are embedded integrated within theguide means and slides and they end into the guides and slidesrespectively by means of contact terminals cooperating one with theother for automatically generating an electrical contact and/or aconduction contact of other power means when the magnetization means 5are in the operative position. The FIG. 3 specifically refers tomagnetization means 5 having at least a layer 305 of magnetizablematerial and an element like a plate for changing and particularly forheating said magnetizable material by supplying an heating electricalcurrent. A portion of the power supply cables for the power supplysignal is denoted by 7 and it extends into the supporting structure 1,201 of the magnetic structure, while the other portion of said cablesextends into the slides 204 and reaches the heating plate 205. Each oneof said cables 6 and 7 ends by a contact terminal element 108, 208. Suchcontact terminal elements 108, 208 are part of automatic electricalconnectors 8 and are placed along the sliding guides 104 and along theslides 204 respectively provided on the magnetization means 5 in suchpositions that they are in the automatic electrical contact conditionone with the other when the magnetization means 5 are in the mounted andoperative position magnetizing the corresponding magnetic pole 2.

From what said above it is clear the great simplicity in operating forrecovering or restoring the magnetization condition of the layer 202 ofsuperconducting bulk material of each magnetic pole 2 upon the change orinterruption of said magnetization condition. Even in the case of heavymagnetization means 5 the shape of the magnetic structure, the structureof the magnetic poles 2, the provision of the insertion and extractionmeans 4 in the form of guides 104 and slides 204 combinations and alsothe shape of the magnetization means 5 allow simple motorized means forpositioning and mounting in place, as well as for extracting themagnetization means 5 from the magnetic structure to be provided, thusallowing the magnet to be always regenerated, in a simple way and withlittle economic and time burden.

FIGS. 2 and 3 show a particular and complicated embodiment of themagnetization means which however is not strictly necessary. Thetechnical effect is achieved even by providing only at least the twolayers 305 and 205 and that is the layer of magnetizable material andthe plate of the heating/cooling means or of means intended to modifythe magnetic order of the magnetizable material.

Such as shown it is possible to provide a layer made of insulatingmaterial 405 particularly thermal insulating material between thesurface of the magnetic pole 2 and the surface faced thereto of themagnetizable material layer 305 of the magnetization means.

Again according to a further improvement in combination with at leastthe two layers 305 and 205 and that is the layer of magnetizablematerial and the plate of the heating/cooling means or of means intendedfor changing the magnetic order of the magnetizable material and/or alsoin the presence of the layer 405 of insulating material it is possibleto provide a layer 105 of permanently magnetized material generating aninitial magnetization condition of the magnetizable layer 305, whichinitial magnetization condition is modified by the thermal wavesgenerated by the temperature changing means 205.

FIGS. 4 and 5 show a side view and a view taken from the front open sideof a variant embodiment of the magnetic structure according to thepreceding figures in that the magnetic structure has a C-shaped orreversed U-shaped cross section, one of the four side walls 201 beingomitted.

Since the structure shown in FIGS. 4 and 5 is not provided with one ofthe vertical walls 201 to which the two sliding guides 104 for theremovable mounting means 4 of the magnetization means 5 were secured inthe variants of FIGS. 1 to 3, in such variant, said guides 104 aresupported by ribs 304 arranged along two opposite side edges of eachmagnetic pole 2 and project perpendicularly to the surface of thecorresponding magnetic pole 2 faced towards the center of the receivingcavity from the corresponding horizontal wall 101 of the supportingstructure 1, which wall supports also the corresponding magnetic pole 2.At the free end of said ribs they support each one a guide 104 intendedto cooperate with the extractable magnetization means 5. As regards themagnetic poles 2 and/or the magnetization means 5 and as regards themeans for removably mounting the magnetization means 5 and the supplycables 6, 7 with automatic connectors 8, such elements and means aremade according to one or more of the variants, combinations orsub-combinations disclosed with reference to the variant embodiment ofFIGS. 1 to 3.

The fact of using a structure with three open sides such as the one ofFIGS. 4 and 5, and as it is known for such structures, at one of theopen sides of the supporting structure and particularly the front sideopposite to the single wall 201 perpendicular to the surfaces of themagnetic poles 2 faced towards the center of the receiving cavity, thestructure has a further additional magnetic pole 2′ interposed betweensaid main magnetic pole 2 and the end of the side 101 of the supportingstructure at the open front side. A secondary pole of this type is knownin EP921408. In this example the secondary pole can be made according toone or more of the embodiments described in EP921408 and like the mainpole 2 as disclosed in the preceding examples and that is by using asuperconducting bulk material for the magnetized layer 202 and byproviding construction structures and magnetization means with all thearrangements of the combinations and sub-combinations described in thepreceding examples for the main magnetic poles 2.

Again according to a further variant embodiment as provided anddescribed in more details in EP921408, between each main pole 2 and theadjacent secondary pole 2′ it is possible to provide a simple gap suchas shown also in FIG. 4 or such gap can receive an element made ofmagnetized material having a particular magnetization direction. Evensuch element can be a conventional permanent magnet or can be composedof superconducting bulk material and can have one or more of the abovecharacteristics described for the main magnetic poles 2 and for theassociated magnetization means.

FIG. 6 shows again a further variant as regards the construction whichallows the magnetization means 5 to be firmly mounted together with themagnetic poles 2 on the supporting structure 1. Although such figureshows only the embodiment provided of a supporting structure with anannular cross-section, i.e. closed on itself and similar to the one ofFIGS. 1 to 3, the characteristics of such variant apply also to astructure with three open sides such as the one of FIGS. 4 and 5.Moreover the described variant can be provided in combination with oneor more of the combinations of the characteristics or sub-combinationsof characteristics described with reference to the preceding FIGS. 1 to5.

Magnetization means 5 are housed into an housing recess 301 providedbackwardly of each corresponding magnetic pole 2, i.e. they areoverlapped on the side of the magnetic pole 2 opposite to the center ofthe receiving cavity.

In this case, means for controlling the temperature of the layer 202 ofmagnetized material denoted by 102 are provided along at least two ofthe edges of the superconducting material layer 202 which edges areperpendicular to the open sides of the receiving cavity and at least anopen side of the recess 301 for the magnetization means 5.

Similarly to what described above, the magnetization means 5 have atleast one layer 305 of magnetizable material together withheating/cooling means 205 or means intended to, change the magneticorder of the magnetizable material and possibly with the insulatinglayer 405 and/or the layer 105 of magnetized material.

Though the present variant embodiment is intended to allow themagnetization means 5 to stay firmly into the magnetic structure of theMRI apparatus, it can be advantageous also in this case to provideremovable mounting means 4 that can be made according to one or more ofthe combinations or sub-combinations of the characteristics describedpreviously with reference to the variant embodiments of FIGS. 1 to 5,for example for carrying out maintenance or repair/replacementoperations.

Moreover with reference to all the embodiments of the present figuresand of the ones described below, it has to be noted that the supportingstructure 1 can be completely or partially a yoke or it can supportconstruction elements of a yoke reclosing the magnetic field generatedinto the receiving cavity by the two magnetic poles 2 and/or also bypossible additional poles 2′.

Again according to a further variant embodiment by using layers 202 ofsuperconducting bulk material which are “permanently” magnetized as longas the temperature of said material is kept below the critical one, thepossibility of generating different magnetizations in different regionsof one element of said superconducting bulk material has been found.This is very important as regards the present MRI apparatus. In thenuclear magnetic resonance diagnostic imaging it is very important forthe static magnetic field acting for generating a reference isotropy forthe orientation of nuclear spins to be homogeneous as much as possibleinside the receiving cavity of the body of the patient or a partthereof. Generally due to aberrations caused by the finite dimensions ofthe magnetic structure, the volume wherein such homogeneity isguaranteed is a sub-volume of the overall volume of the receiving cavitysuch as a central spherical region whose extension is smaller than theone of said receiving cavity. For guaranteeing that optimization stepsare often necessary which are called as shimming and which are complex,difficult, relatively long and expensive.

Moreover as it is proved in the prior art the magnetic field generatedby an element of superconducting bulk material that is permanentlymagnetized in the superconducting condition is never homogeneous withreference to the distribution of the field strength along a surface ofsaid element. Thus particularly the magnetic field generated by amagnetic pole 2 of the type described above would not be homogeneous andit would require a very complex and difficult optimization operation.

In this case as an improvement the invention provides to use thepossibility of modifying the magnetization locally and for differentpartial regions of an element of superconducting bulk material. Thisallows the local dishomogeneities of the magnetic field along theextension of the surface of the magnetic pole 2 faced towards the cavityto be compensated, at least up to a certain variation threshold of saidmagnetic field generated by said magnetic pole 2 working withsuperconducting bulk material as permanent magnet.

The invention substantially provides two different variants. Accordingto a first variant, each magnetic pole has a layer 202 of magnetizedmaterial composed of superconducting bulk material and it is made of anassembly of adjacent blocks or cards 402 made of said superconductingbulk material. Blocks and cards are arranged side by side along twodirections perpendicular one with respect to the other and parallel tothe surface of the magnetic pole 2 such to make an array of said blocksor said cards 402. Each block or each card 402 can be individuallymagnetized depending on the field strength desired in the locationcorresponding thereto. Due to that the distribution of the magneticfield in the perpendicular extension directions of the magnetic pole 2can be modified.

An alternative solution provides the same effect to be achieved byproviding a layer 202 of magnetized material as one piece i.e. acontinuous one, whose face parallel to the surface of the magnetic pole2 faced towards the center of the receiving cavity is ideally dividedinto adjacent sectors according to two directions perpendicular one withthe other and parallel to said surface and making an array of adjacentsectors.

The plan view of the face of the layer 202 of magnetized material forboth the above variants is shown in FIG. 15. In the first case thesquares represent the blocks or cards 402 and the partitition lines arethe contact surfaces of the block or cards 402. In the second case thesquares represent the sectors each one being magnetizable differentlyfrom the other sectors and the lines separating them are the boundarylines of said sectors.

FIGS. 7 to 12 schematically show several variant embodiments of theconstruction of magnetic poles 2 and of the magnetization means 5 withreference to the two principle variant embodiments described above.

FIGS. 7 and 11 show the magnetic pole 2 and the magnetization means intheir minimum composition and that is: the magnetic pole 2 comprises alayer 202 of superconducting bulk material intended to form the layer ofmagnetized material and the means 102 for cooling the temperature belowthe critical one, i.e. the transition temperature in the superconductingcondition; magnetization means are composed of the layer 302 ofmagnetizable material and of means 102 for generating the thermal wave.Obviously it is possible to provide as described above also furtherlayers and i.e. for the magnetic poles 2, the plate of permeable orferromagnetic material and for the magnetization means the insulatinglayer and/or the layer of permanently magnetized material denoted by 405and 105 in FIGS. 1 to 5.

FIG. 7 shows the variant wherein the layer 202 of superconducting bulkmaterial is continuous, while the individual sectors are differentlymagnetized by means of magnetization means 5 dedicated to eachindividual sector and which can be alternatively simultaneously orsequentially operated. Therefore each sector of the layer ofsuperconducting bulk material is associated to a dedicated magnetizationunit composed of at least a block 605 of magnetizable material and ofmeans 505 for generating the thermal wave. The magnetization unit has anoperative magnetization surface whose shape and size substantiallycorrespond to the ones of the associated sector of the layer ofsuperconducting bulk material and it overlaps the surface of said sectorfaced towards the center of the receiving cavity.

Individual magnetization units are mounted into a frame having sideslides 204 engaging into sliding guides 104 of the removable mountingmeans 4 according to one or more of the variants previously described.

FIG. 8 shows a variant of FIG. 7 wherein even the layer of magnetizedmaterial of the magnetization means is continuous, while on the contrarymeans 505 for generating the thermal wave are composed of elements eachone being dedicated to a sector into which the layer 202 ofsuperconducting bulk material of the magnetic pole 202 is divided.Therefore even in this case it is possible to magnetize differently andindividually each sector of the layer 202 of superconducting bulkmaterial. The individual magnetization unit is composed of theindividual means 505 generating the thermal wave having an operativemagnetization surface whose shape and size substantially correspond tothe ones of the associated sector of the layer of superconducting bulkmaterial and it overlaps the surface of said sector faced towards thecenter of the receiving cavity.

The variant of FIG. 9 is similar to the variant of FIG. 7 as regards theconstruction of the magnetization means. However in this case the layerof superconducting bulk material is composed of blocks or cards 402. Themagnetization units have an operative magnetization surface whose shapeand size substantially correspond to the ones of the associated block orthe associated card 402 of the layer of superconducting bulk materialand they overlap the surface of said block or of said card faced towardsthe center of the receiving cavity.

The variant of FIG. 10 is similar to FIG. 8 as regards the constructionof the magnetization means and of the individual magnetization unitsconstituting it. It differs from the variant of FIG. 8 in that the layerof superconducting bulk material is made of cards or blocks 402. Themagnetization units have an operative magnetization surface which is theone of the individual means 505 generating the thermal wave and whichoperative surface has a shape and a size substantially corresponding tothe ones of the associated block or the associated card 402 of the layerof superconducting bulk material and they overlap the surface of saidblock or said card faced towards the center of the receiving cavity.

The variant of FIG. 11 provides a layer of superconducting bulk materialmade of cards or blocks 402 like the embodiments of FIGS. 9 and 10.However, in this case, magnetization means are composed of a singlemagnetization unit 605 made of magnetized material and of means 505generating the thermal wave like the embodiment according to FIG. 7. Themagnetization units have an operative magnetization surface whose shapeand size substantially correspond to the ones of each block or card 402of the layer of superconducting bulk material and it overlaps thesurface of one block or one card faced towards the center of thereceiving cavity due to the fact that it can be moved from one block toanother one or from one card to another one.

Besides complex embodiments that can provide robotized arms or othersimilar means, an embodiment providing a frame 705 having along the twoopposite sides the slides 204 for sliding into the guides 104 integralwith the supporting structure is a preferred one since it allows themagnetization means to be extracted and mounted by means of removablemounting means made as the ones denoted by 4 and described and shownwith reference to FIGS. 1 to 10. The frame subtends a surface parallelto the surface of the magnetic pole 2 faced towards the center of thereceiving cavity and it bears means 9 and 10 for moving saidmagnetization unit 505, 605 in two directions perpendicular one to theother and parallel to said surface of the magnetic pole faced towardsthe center of the receiving cavity.

The first translating means 9 permit a translation perpendicular to theone of the guides 104 for inserting and extracting the frame 705 intothe receiving cavity and are made of a transversal guide 109 that isstationary and connects the two parallel sides to the slides 204 and tothe sliding guides 104. On said transversal guide 109 a slide 209 isslidably mounted in the longitudinal direction of said transversal guide109. The slide 209 bears in turn a guide 110 for the translating means10 wherein a slide 201 is slidably mounted. The slide 210 moves alongthe guide 110 in a direction parallel to the guides 104 forinserting/extracting the frame 705 into the receiving cavity andperpendicularly to the guide 109.

Thanks to the above the magnetization unit can be moved from one of theblocks or cards 402 to another one by providing each one of them to besequentially differentially magnetized.

As an alternative to what shown in FIG. 11 it is possible to provide thelayer of superconducting bulk material to be continuous and to providethe movement of the magnetization unit to lead to the individualmagnetization of the individual sectors into which the continuous layerof superconducting bulk material is divided.

FIG. 12 shows again a further embodiment wherein the magnetic pole 2 hasa continuous superconducting bulk layer 202 and moreover also a layer offerromagnetic or permeable material 302, while the magnetization meansare made such as shown and described in FIG. 7. The plurality of theshown and described combinations reveals and makes acceptable the factthat even other combinations are possible above all as regards theprovision or the absence of layers and/or of structural elements of themagnetization means and of the magnetic poles with reference to thevariants described above for the magnetization means and for themagnetic poles.

FIGS. 13 and 14 show some of the variants of FIGS. 7 to 12 andparticularly of FIGS. 7, 8, 9 and 11 which are mounted into a supportingstructure 1 of the type with three open sides. Said variants are shownfor one of the poles of said magnetic structures respectively in orderto make clear how each one of them is combined with a supportingstructure 1.

FIG. 13, by broken lines, shows the omitted wall 201 perpendicular tothe magnetic poles 2 in order to show also the possibility of thevariant with an annular cross-section i.e. the structure having only twoopposite openings.

As regards the variants of FIGS. 7 to 15, it is important to note howthe power supply cables separated from the magnetization units 505, 605can be integrated into the movable mounting means 4 like what describedwith reference to the examples of the preceding FIGS. 2 to 4 automaticcontact connectors 8 being similarly provided.

FIG. 15 shows the importance of the possibility of providing a differentand separate magnetization for the sectors of the surface of themagnetic pole 2 and particularly of the layer of superconducting bulkmaterial.

In this case it has to be noted that such local magnetizationdiversified per sectors helps for optimizing the homogeneity of thestatic magnetic field into the receiving cavity and throughout theextension of the magnetic poles 2, but it can help also for avoidingauxiliary or secondary poles to be provided such as shown with referenceto FIG. 3 which can be replaced by or whose function can be taken bysub-assemblies of sectors of the magnetic poles, whose magnetizationdifferent from the other sectors of the magnetic pole that is of thelayer of superconducting bulk material is such to generate the effect ofa secondary or auxiliary pole, particularly a secondary or auxiliarypole compensating the field aberrations at the openings of the receivingcavity such as taught by the document EP921408. The differentmagnetization can involve the magnetization strength, but also themagnetization direction, thus it is possible to obtain also the effectof magnetized material forming inserts separating the main and auxiliaryor secondary poles according to the teaching of the document EP921408.

The invention claimed is:
 1. MRI apparatus comprising: a magneticstructure defining a cavity for receiving a body under examination or apart thereof, means for generating a magnetic field inside said cavity,means for causing the body under examination or a part thereof to emitnuclear magnetic resonance signals, means for receiving said nuclearmagnetic resonance signals, and an electronic processing unit, saidmeans receiving the nuclear magnetic resonance signals beingelectrically connected thereto; wherein the cavity includes at least anaccess opening through which a body under examination or a part of saidbody can enter said cavity; and wherein said means for generating themagnetic field comprise one or more elements made of permanentlymagnetized material of the so-called superconducting bulk material type,wherein magnetization currents are firmly trapped, in combination withmeans for keeping said magnetization condition of said superconductingbulk material.
 2. Apparatus according to claim 1, wherein said means formaintaining the magnetization condition of said superconducting bulkmaterial comprise means for keeping the temperature of saidsuperconducting bulk material below the critical temperature thereof. 3.Apparatus according to claim 1, wherein the permanently magnetizedsuperconducting bulk material is of the type-I or type-II.
 4. Apparatusaccording to claim 1, wherein the superconducting bulk material iscomposed of a medium critical temperature material and particularly ofMgB₂.
 5. Apparatus according to claim 1, wherein the superconductingbulk material is composed of a material with a critical temperature from20 to 60, preferably 35 to 50 kelvin degrees.
 6. Apparatus according toclaim 1, wherein the magnetic structure includes at least two poles, andfurther comprising means for magnetizing the magnetized material whichare of the removable type, wherein the magnetic structure and/or thepoles and the magnetization means being provided with mutually removablefastening means.
 7. Apparatus according to claim 6, wherein the mutuallyremovable fastening means for mutually fastening the magnetization meansto the magnetic structure and/or to the poles are composed of acombination of sliding guides and slides or saddles which are slidablyengaged one to the other according to an insertion/extraction directionof the magnetization means in a position coupling with the correspondingpole.
 8. Apparatus according to claim 7, wherein the sliding guides forthe insertion/extraction of the magnetization means are orientedparallel to the axis of the or of one of the access openings of thecavity and they are provided in a position coincident with the passagespan defined by said opening such that said magnetization means areinserted/extracted in and from the position coupling to the permanentlymagnetized material through said access opening.
 9. Apparatus accordingto claim 7, wherein each element made of magnetized material is in theform of a layer like a sheet or plate made of permanently magnetizedmaterial having a predetermined thickness and a predetermined area and apredetermined plan shape and the guides for inserting/extracting themagnetization means are oriented parallel with respect to the surface ofsaid sheet or plate of permanently magnetized material at the side ofsaid sheet or plate and in such a position that with the magnetizationmeans in the inserted condition into said guides, said means overlap thesurface of said sheet or plate made of magnetized material. 10.Apparatus according to claim 9, further comprising magnetization meanscomposed of a supporting frame that can be inserted/extracted by theinsertion/extraction guides and upon which frame there are mounted meansfor supporting a magnetization head having such a size to cooperate onlywith one of the blocks or cards forming a sheet of superconducting bulkmaterial or only with one of the sectors into which the area of a faceof the sheet or plate in the form of one piece of superconducting bulkmaterial is ideally divided and which is movable in two perpendiculardirections alternatively in one position cooperating with a block or acard or with a sector one at a time respectively of the sheet or platein the form of one piece of superconducting bulk material of theplurality of blocks, cards or sectors.
 11. Apparatus according to claim10, wherein the frame has a first slide slidable along one of twoperpendicular directions, which slide is the supporting guide sliding asecond slide that is movable along said first slide in said seconddirection.
 12. Apparatus according to claim 10, wherein themagnetization means are composed of an assembly of individualmagnetization units, each one of which having such a size to magnetize ablock, a card or a sector forming the sheet or plate of superconductingbulk material said magnetization units being mounted one with respect tothe other on a supporting frame according to an order and an arrangementof the relative positions one with respect to the other corresponding tothe relative arrangement one with respect to the other of blocks, cardsor sectors forming the sheet or plate of superconducting bulk materialand such that each one of said magnetization units can be taken in aposition cooperating with a corresponding block, a corresponding card ora corresponding sector simultaneously to the other magnetization unitsand during a single and common motion inserting the assembly of themagnetization units into the insertion or extraction guides. 13.Apparatus according to claim 12, wherein each magnetization unit can beoperated separately from the others, said units being simultaneouslyoperated at different times according to a predetermined sequence. 14.Apparatus according to claim 13, wherein each block, each card or eachsector of the plate or sheet of superconducting bulk material aremagnetized such to achieve a different magnetization one with respect tothe other for generating a predetermined arrangement of the magneticfield throughout the area of said plate or sheet.
 15. Apparatusaccording to claim 6, wherein the magnetic structure comprises asupporting structure to which the poles generating the magnetic fieldare secured.
 16. Apparatus according to claim 15, wherein the supportingstructure is composed of a magnetic yoke or it serves as a yokereclosing the magnetic flux.
 17. Apparatus according to claim 1, whereineach element made of magnetized material is in the form of a layer likea sheet or plate made of permanently magnetized material having apredetermined thickness and a predetermined area and a predeterminedplan shape.
 18. Apparatus according to claim 17, wherein the magneticstructure includes at least two magnetic poles, and wherein on the sidefaced towards the cavity each sheet or plate made of magnetized materialprovided for each magnetic pole is overlapped by a sheet-like orplate-like element made of ferromagnetic material.
 19. Apparatusaccording to claim 17, wherein each sheet or plate made of permanentlymagnetized material is composed of at least a layer of blocks or cardsmade of superconducting bulk material arranged one near the other oneinto a two-dimensional array of blocks or cards made of superconductingbulk material, means for individually magnetizing each block or eachcard of superconducting bulk material being provided.
 20. Apparatusaccording to claim 19, further comprising means for keeping thetemperature of the individual blocks or individual cards ofsuperconducting bulk material below the critical temperature which areseparated for each block or each card or are in common to all the blocksor to all the cards of one sheet or plate made of permanently magnetizedmaterial.
 21. Apparatus according to claim 17, wherein each sheet orplate of permanently magnetized material is as one piece and has a sidefaced towards the cavity which side has a predetermined plan shape and apredetermined area, said area being ideally divided into a plurality ofadjacent sectors like an array of sectors and each one of said sectorsbeing magnetized separately from the other sectors.
 22. Apparatusaccording to claim 1, wherein the magnetic structure comprises a yokeand two magnetic poles, each magnetic pole comprises in turn a layer ofmagnetized material in the form of a plate or sheet and the magneticpoles are parallel and opposite one to the other and are formed at twoparallel and opposite sides of the yoke, while the magnetizations of thelayers of magnetized material being parallel and aligned in the samedirection, substantially perpendicular to the inner faces of the mainmagnetic poles said magnetic structure being further shaped such todefine or enclose a cavity, at least a portion of the volume of saidcavity being a space for receiving at least a part of a body underexamination, at least a portion of said cavity being permeated by astatic magnetic field generated by the magnetic poles and havingspecific strength and homogeneity characteristics said magneticstructure being open at least one side parallel to the static magneticfield.
 23. Apparatus according to claim 22, wherein along the edges ofthe at least one open side, in the proximity of the at least one openside, the magnetic structure further comprises at least two additionalmagnetic poles, each one of said additional magnetic poles comprises alayer of magnetized material and a plate of high-permeability material,said additional magnetic poles extend at least with a substantiallyequal extent as the main magnetic poles and are parallel and oppositeone with respect to the other, said additional magnetic poles beingformed at the two parallel and opposite sides supporting the mainmagnetic poles, while the magnetizations of the layers of magnetizedmaterial of the additional magnetic poles are parallel and aligned inthe same magnetization direction of the layers of magnetized material ofthe main magnetic poles, the layer of magnetized material of theadditional magnetic poles being made of superconducting bulk material.